The present invention pertains generally to digital phase detectors and specifically to a pulse count generator for use therein to permit the phase detection function to be accomplished through a counting process which ensures that all pulses in the two signal pulse trains to be compared, including coincident pulses, are properly counted.
As is well known, digital phase detectors are often used in phase lock loop circuits to provide the correction signal for maintaining correspondence of frequency between the tracking and reference signals. Maintenance of frequency correspondence in the presence of frequency variations in the tracking signal is greatly enhanced when the phase difference between the tracking and reference signals remains within the phase lock range of the phase detector, viz. the scope of operation within which the integrated output of the phase detector remains a linear function of the phase difference. Most, if not all, digital phase detectors afford a phase lock range of only 360.degree. which permits the tracking signal to slip or advance by only one cycle before the detector linear range is exceeded. By extending the detector linear range, its usefulness and versatility can be materially improved by increasing the speed with which phase lock is achieved and eliminating or at least reducing the number of excursions out of phase lock which degrade performance and create frequency overshoots which can detrimentally affect other nearby equipment.
One of the problems in attempting to extend the phase lock range in a phase detector relates to the need for accounting for all pulses in the tracking and reference signals to achieve reliable operation. All pulses must be properly processed irrespective of their time of occurrence. Thus, coincident pulses in the tracking and reference signals which occur as the tracking signal shifts with respect to the reference signal must be registered and accounted for to avoid introducing errors into the phase lock loop operation. Since commercial up/down digital counters cannot count both ways at the same time and the use of two separate counters for overcoming the coincident pulse problem would only materially add to the cost of a digital phase detector in an already highly competitive market, there may not have been sufficient financial incentive to justify attempts at extending the phase lock range of digital phase detectors.
With the foregoing in mind, it is a primary object of the present invention to provide a pulse count generator for supplying mutually exclusive counting pulses corresponding to the pulses in two separate pulse trains.
It is a further object of the present invention to provide such a pulse count generator having a simple and inexpensive design so that it may be easily and economically used in a digital phase detector for extending its phase lock range to enhance phase lock loop operation.
These, as well as other objects, and the means by which they are achieved may be best appreciated by referring to the Detailed Description of the Invention which follows hereinafter together with the appended drawings.